Photo- and electro-luminescence of three TADF binuclear Cu(i) complexes with functional tetraimine ligands

Article abstract of DOI:10.1039/c7tc00443e

Three new binuclear cuprous complexes with similar tetraimine ligands [Cu₂(pytzph)(POP)₂](BF₄)₂ (1), [Cu₂(pytzphcf)(POP)₂](BF₄)₂ (2) and [Cu₂(pytzphcz)(POP)₂](BF₄)₂ (3), (pytzph = 6,6′-(1-phenyl-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine), pytzphcf = 6,6′-(1-(4-(trifluoromethyl)phenyl)-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine), pytzphcz = 9-(4-(3,5-bis(6-methylpyridin-2-yl)-1,2,4-triazol-1-yl)phenyl)-carbazole and POP = bis[2-(diphenylphosphine)phenyl]ether), have been synthesized and characterized in order to compare the different effects of substituent groups on the photoluminescence (PL) and electroluminescence (EL) properties. These complexes exhibit highly efficient green thermally activated delayed fluorescence (TADF) with short decay times (5.5-16 μs) and high photoluminescence quantum yields (up to 79%) at room temperature in the solid form. These complexes have essentially identical emission energy. However, the influence of the substituents on the photoluminescence and electroluminescence efficiencies is evident. Complex 3 with the carbazole group shows the highest efficiency in terms of both PL and EL, exhibiting an EQE of 8.3%, a CE of 27.1 cd A^-1 and a peak brightness of 2525 cd cm^-2 in the solution-processed OLED, while complex 2 with a trifluoromethyl appendage exhibits poorer quantum efficiency than the others.

Full text of DOI:10.1039/c7tc00443e

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Materials Chemistry C
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Photo- and electro-luminescence of three TADF
binuclear Cu(I) complexes with functional
tetraimine ligands†
Cite this:DOI: 10.1039/c7tc00443e
ab
ab
a
a
Ling Lin,
Dong-Hui Chen,
Rongmin Yu, * Xu-Lin Chen,
a
ab
a
a
Wen-Juan Zhu,
Can-Zhong Lu
Dong Liang,
Jian-Fei Chang,
Qing Zhang
and
a
*
Three new binuclear cuprous complexes with similar tetraimine ligands [Cu
2
(pytzph)(POP) ](BF
2 4
)
2
(1),
0
[Cu (pytzphcf)(POP) ](BF )
2 2 4 2
(2) and [Cu (pytzphcz)(POP) ](BF )
2 2 4 2
(3), (pytzph = 6,6 -(1-phenyl-1,2,4-triazole-
0
3,5-diyl)bis(2-methylpyridine), pytzphcf
=
6,6 -(1-(4-(trifluoromethyl)phenyl)-1,2,4-triazole-3,5-diyl)bis-
(2-methylpyridine), pytzphcz = 9-(4-(3,5-bis(6-methylpyridin-2-yl)-1,2,4-triazol-1-yl)phenyl)-carbazole and
POP = bis[2-(diphenylphosphine)phenyl]ether), have been synthesized and characterized in order to
compare the different effects of substituent groups on the photoluminescence (PL) and electroluminescence
(EL) properties. These complexes exhibit highly efficient green thermally activated delayed fluorescence
(TADF) with short decay times (5.5–16 ms) and high photoluminescence quantum yields (up to 79%) at room
Received 26th January 2017,
Accepted 2nd April 2017
temperature in the solid form. These complexes have essentially identical emission energy. However, the
influence of the substituents on the photoluminescence and electroluminescence efficiencies is evident.
Complex 3 with the carbazole group shows the highest efficiency in terms of both PL and EL, exhibiting an
DOI: 10.1039/c7tc00443e
ꢀ
1
ꢀ2
EQE of 8.3%, a CE of 27.1 cd A and a peak brightness of 2525 cd cm in the solution-processed OLED,
rsc.li/materials-c
while complex 2 with a trifluoromethyl appendage exhibits poorer quantum efficiency than the others.
yields have been obtained and shown to have attractive perfor-
mance in OLEDs. These cuprous complexes are proved to be
Introduction
Since Tang and VanSlyke first fabricated ultrathin multilayer thermally activated delayed fluorescence (TADF) materials in
1
3
organic electroluminescent diodes, the application of OLEDs which the energy gap between the S and T states is small.
1
1
in planar displays and lighting has attracted more and more Theoretically, they are able to harvest all the singlet and triplet
attention. During the last few decades, Ir(III), Pt(II), or Os(II) excitons through reverse intersystem crossing from the lowest
1 1
based complexes used as phosphorescent organic light-emitting excited triplet state (T ) to the lowest excited singlet state (S ) by
3b,4
diodes (PHOLEDs), which are able to take full advantage of both thermal activation.
triplet and singlet excitons to achieve 100% internal quantum Cuprous complexes are usually used as dopants in emitting
efficiency, have received extensive academic and commercial layers. In order to design suitable complexes to obtain highly
2
interests. However, due to the high price and limited availability efficient OLEDs, one way is to improve the photoluminescence
of these noble metal complex materials, more and more effort has efficiency of Cu(I) complexes, mainly by means of enhancing
been made to discover new alternatives. Cuprous complexes are steric hindrance in ligands to minimize geometry distortion in
some of the potential candidates. Although the heavy atom effects the excited state. And thus, the non-radiative deactivation
5
of copper are weaker than those of the noble metals mentioned process can be restrained. Another method is to introduce
above, plenty of emissive Cu(I) complexes with high quantum functional groups at ligands to improve the hole injection
5a,6
or electron transmission properties.
For example, carbazole
a
substituent groups can promote the hole transportation
abilities of OLEDs owing to their rich electron properties.
Key Laboratory of Design and Assembly of Functional Nanostructures,
Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, China.
E-mail: rongminyu@fjirm.ac.cn, czlu@fjirsm.ac.cn
3
e,7
This is also a way to raise the utilization rate of excitons,
besides matching the energy levels between charge transfer
layers. However, reports on different characteristics of emissive
cuprous complexes with oppositely functional substituent
groups are rare.
b
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Electronic supplementary information (ESI) available: Supplementary computa-
†
tional and experimental data. CCDC 1528952–1528954. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c7tc00443e
This journal is ©The Royal Society of Chemistry 2017
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Optimization of the device fabrication is another hotspot in of compounds 1–3 in CDCl
Journal of Materials Chemistry C
3
display well-resolved peaks. They
OLED research. Solution processing is one of the promising show a single sharp set of signals for the tetraimine ligands
methods for preparing commercialized devices, owing to its that can be assigned ambiguously as referenced to the spectra
potential in low-cost and large area manufacturing techno- of the free ligands. The resonances of the two methyl groups are
7
a
logies, e.g. spin coating, inkjet printing and screen printing.
singlet and sharp peaks at about 2.3 and 1.9 ppm, displaying an
It has been widely used in preparing emitting layers from upfield shift of 0.3 ppm as compared to those in the free
3
1
various materials, such as polymeric materials and many small ligands. In the P NMR spectra of 1–3, two singlet resonances
molecular materials including pure organic materials and ionic at about ꢀ12.5 to ꢀ18.4 ppm are observed, which is consistent
3
e,8
metal complexes.
with the expectation from the structures as shown in Fig. 1.
In this work, we report three binuclear cationic cuprous com- These complexes also display high thermal stability without
plexes [Cu (pytzph)(POP) ](BF ) (1), [Cu (pytzphcf)(POP) ](BF ) degradation until 325 1C as shown by thermal gravimetric
2) and [Cu (pytzphcz)(POP) ](BF ) (3), bearing different func- analysis (Fig. S2, ESI†).
2
2
4 2
2
2
4 2
(
2
2
4 2
0
tional tetraimine ligands and bisphosphine (pytzph = 6,6 -(1-
The crystal structures of 1–3 consist of cationic Cu(I) com-
phenyl-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine), pytzphcf = plexes and anionic boron tetrafluoride and dichloromethane as
0
6
,6 -(1-(4-(trifluoromethyl)phenyl)-1,2,4-triazole-3,5-diyl)bis(2- interstitial molecules. The ORTEP diagrams of their cations are
methylpyridine), pytzphcz = 9-(4-(3,5-bis(6-methylpyridin-2-yl)- shown in Fig. 2, and selected bond lengths and angles are listed
,2,4-triazol-1-yl)phenyl)-carbazole and POP = bis[2-(diphenyl- in Table S2 in the ESI.† In each cation, the Cu(I) centers are
1
phosphine)phenyl]ether) (Fig. 1). All complexes emit strong four-coordinate with a distorted tetrahedral geometry, the
green TADF. Their photo-physical properties and electrolumi- tetraimine ligand is chelated and bridged to the Cu(I) centers,
nescence behaviours are studied in detail and compared experi- and the POP ligand is chelated terminally to the Cu(I) centers to
mentally and theoretically.
complete the coordinated sites. The Cu–P and Cu–N bond
lengths in these complexes are nearly identical, as shown in Table
S2 in the ESI,† and are close to those found in typical mono-
+
Results and discussion
Synthesis and characterization
nuclear [Cu(PP)(NN)] complexes, such as [Cu(dmp)(POP)]BF
4
5b
(dmp = 2,9-dimethyl-1,10-phenanthroline). From a comparison
of the N–Cu–N and P–Cu–P angles at different Cu(I) centers, the
steric influence of the aromatic substituents in the tetraimine
ligands can be seen. The N–Cu(1)–N and P–Cu(1)–P angles of
Cu(1) that is trans to the substituents are in the range of
The tetraimine ligands were synthesized with a moderate over-
all yield and their synthetic routes are shown in the ESI†
(Fig. S1). Binuclear cuprous complexes 1, 2 and 3 were obtained
from the reaction of [Cu(CH CN) ]BF , POP and the corres-
3
4
4
78.78(8)–79.5(2)1, and 118.22(3)–118.57(3)1, respectively. How-
ponding tetraimine ligands. Fig. 1 shows the molecular struc-
tures of three complexes. Crystals suitable for X-ray studies
were obtained by layering diethyl ether onto the solution of the
ever, the N–Cu(2)–N and P–Cu(2)–P angles of Cu(2) that is cis
to the substituent, are 76.1(2)–78.38(7)1 and 111.22(6)–113.18(3)1,
respectively. Moreover, the dihedral angle between the planes of
N–Cu(1)–N and P–Cu(1)–P is 89.21(6), 88.9(1) and 89.40(7)1 for
complexes 1–3, respectively, while the dihedral angle between the
planes N–Cu(2)–N and P–Cu(2)–P is 80.47(5)1, 76.9(1)1 and
2 2
complexes in CH Cl . The crystals of these complexes quickly
lose their crystallinity after being exposed to the air. However,
the complexes are stable both in powders and in solution. Their
1
31
purities were characterized by H NMR, P NMR, PXRD and
80.05(7)1 for 1–3. This indicates that the tetrahedral geometry of
1
elemental analysis (Fig. S4 and S5, ESI†). The H NMR spectra
the Cu(2) atoms in these complexes is more distorted. The
decrease of the N–Cu(2)–N and P–Cu(2)–P angles as compared
to the N–Cu(1)–N and P–Cu(1)–P angles, and the more distorted
geometry around Cu(2) are ascribed to the steric effect of the
substituents that are closer to Cu(2). From the structure shown in
Fig. 2, we can also see that the phenyl substituent is not coplanar
with the five-membered triazolate ring. In the tetraimine ligand,
the dihedral angle between the phenyl ring and the triazolate
plane is 83.6(1)1 for 1, 84.1(2)1 for 2 and 84.2(1)1 for 3. This
indicates that the phenyl ring is not in conjugation with the left
side of the tetraimine moiety. Therefore, the electronic influence of
3
the electron-withdrawing group (CF ) in 2 and the electron-
donating group (carbazole) in 3 on the electronic structures of
these complexes is very limited. This is consistent with the similar
energy of the emission of these complexes (vide infra). From the
structures we can also see that the methyl groups in the tetraimine
ligand are very close to the Cu(I) centers. These methyl groups are
designed specifically to provide steric protection around the metals.
4a,9
Fig. 1 Drawing of molecular structures of the cuprous complexes.
This is an important consideration for increasing the PLQY.
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Fig. 2 ORTEP diagrams of 1 (left), 2 (middle) and 3 (right) with thermal ellipsoids at the 50% probability level. Solvent molecules, the anions, H atoms and
some phenyl C atoms of the POP ligands are omitted for clarity.
Photophysical properties
mainly distributed over the Cu metal and phosphine ligands and
the LUMO has main contributions from the tetraimine ligand. As
for complex 3, the electron density of the HOMO is concentrated in
the carbazole part and the LUMO centers on the other part of the
tetraimine ligand. According to the orbital composition analyses
0,11 and orbital transition analyses (see Fig. 4, Tables S6 and S7 in the
ESI†), the lowest lying transitions of 1 and 2 mainly consist of
LLCT and MLCT characters, while those of 3 exhibit substantial
ILCT characters. The uncommon performance of 3 results from
the strong electron-donating ability of the carbazole group.
Fig. 3 shows the UV-vis spectra of complexes 1–3 in dichloro-
methane, and the spectra of the free ligands are present as well
for comparison. Similar to many mononuclear four-coordinated
+
[
Cu(N,N)(P,P)] complexes in which N,N represents a diimine
1
chelating ligand and P,P is a bisphosphine chelating ligand,
these binuclear cuprous complexes exhibit intense absorption
peaks below 320 nm, which can be assigned to the spin-allowed
p–p* transitions of the tetraimine and bisphosphine ligands. At a
longer wavelength, complexes 1 and 2 show weak absorption
bands that are attributed to phosphine ligand–tetraimine ligand
charge transfer (LLCT) and metal–imine ligand charge-transfer
The spectra and selected photophysical data of these
cuprous complexes in degassed CH Cl are shown and listed
2
2
in Fig. 3, Fig. S6 and Table S3 (ESI†). For comparison, the
emission properties of the free tetraimine ligands were also
(MLCT) transitions. But the absorption bands of complex 3 in
the region above 320 nm, which are also observed in the
pytzphcz ligand, are mainly dominated by intraligand charge
transfer (ILCT) transitions. Besides, the bands of 3 below 360 nm
are more intense than those of 1 and 2, which is also owing to
the ILCT transitions of the tetraimine ligand.
The analyses are supported by Density Functional Theory (DFT)
and Time-Dependent DFT (TD-DFT) calculations (see Fig. 4 and
Table S6, ESI†). For 1 and 2, the electron density of the HOMO is
measured in degassed CH
spectra and the selected data of the emissive properties are
listed in Fig. S6 and Table S4 in the ESI.† In CH Cl , the
2 2
Cl at room temperature. The
2
2
tetraimine ligands exhibit near ultraviolet to blue emission
under UV light. The pytzph ligand gives deep blue emission
with lmax = 421 nm, t = 7 ns and PLQY = 2.4%. The pytzphcf
ligand displays near-ultraviolet light with lmax = 370 nm, t = 4
ns and PLQY = 2.8%. The pytzphcz ligand has a blue emission
with lmax = 445 nm, t = 3 ns, and PLQY = 27%. The influence of
the electronic properties of CF
emission properties of the ligands is evident. The electron-
withdrawing CF group leads to a blue shift of the emission
3
and carbazole groups on the
3
maximum, while the electron-donating carbazole group results
in a red shift of the emission maximum. Unlike the free
ligands, the studied Cu(I) complexes exhibit emissions with
nearly identical emission maxima. In degassed CH
solution, the emission peak maxima lie at 513 (1), 509 (2) and
10 nm (3). The Stokes shifts calculated from the differences
2 2
Cl
5
between individual excitation and emission spectral maxima
are 143 nm for 1, 139 nm for 2 and 140 nm for 3, respectively.
The photoluminescence quantum yields of 1, 2 and 3 at room
temperature are between 6% and 9%. In the PMMA films of the
complexes with 20% weight concentration, the emission peak
maxima lie at 514 (1), 506 (2), and 512 nm (3). The emission
quantum yields are 20, 16, and 26% for complexes 1–3, respec-
tively. Higher emission quantum yields were obtained from
Fig. 3 Absorption spectra (left) of the cuprous complexes as well as free
ꢀ5
ligands in degassed CH
right) of the cuprous complexes in degassed CH
temperature.
2
Cl
2
(c E 2 ꢁ 10 M) and the emission spectra
(
2
2
Cl measured at room
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Fig. 4 Frontier molecular orbitals for complexes 1–3 from DFT calculations (left). Redistribution of electron densities of the lowest singlet excited state
middle) and the lowest triplet excited state (right) from TD-DFT calculations.
(
PYD2 films. The PLQYs of these complexes in PYD2 films with
0% concentration are 45, 37 and 59% for 1, 2 and 3, respec-
2
tively (see Table S5 in the ESI†). The observed enhancement of
the quantum yields in the films compared to that in the
solutions could be explained by the rearrangements in the
excited states that are hampered in a more rigid environment.
The emission spectra of complexes 1–3 in the solid form at
298 K and 77 K are shown in Fig. 5, and their photophysical
data are summarized in Table 1. At 298 K, these Cu(I) complexes
emit efficient green emission with the emission peaks at 509(1),
519(2) and 503(3) nm. The decay times are in the range of 6–16
ms. Interestingly, complex 3 exhibits a significantly higher PLQY
than the other studied complexes, which indicates that the
introduction of the carbazole group in the imine ligand is Fig. 5 Emission spectra of the cuprous complexes in the crystal state at
beneficial to increase the PLQY. With decreasing temperature, different temperatures.
the decay times of the complexes increase hugely, meanwhile
the PLQYs of these complexes also increase significantly. The
lower PLQYs of the complexes are mainly attributed to the non- emission peaks upon heating. The observed changes in the radiative
nr
ꢀ1
radiative rate constant [k = (1 ꢀ FPL)t ] that increases by 48, rate and the spectral shift of the emission peaks upon temperature
1
2
3
38 and 45 times for 1, 2 and 3, respectively. At 77 K, the decay change can be explained by the emission mechanism of TADF.
time amounts to 359 ms and is therefore assigned to originate
For further understanding the nature of the excited states,
r
ꢀ1
from the triplet state T . From k = FPL
t
, the radiative the decay times of the complexes were measured in the tem-
1
constants are calculated. When the temperature rises from perature range of 77–378 K, and summarized in Fig. 6. According
r
ꢀ1
7
7 K to 298 K, the radiative rate constants (k = FPL
t ) increase to the TADF mechanism, the emission of 1–3 arises from two
by a factor of 19 (for 1), 12 (for 2) and 36 (for 3). A large increase inter-convertible excited states in thermal equilibrium, i.e., S
1
1 1
in the radiative rates is accompanied by the blue shifts of the and T . Supposing a fast thermal equilibrium between the S and
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Table 1 Photophysical data of 1–3 in the crystal state
2
98 K
77 K
a
b
c
d
r e ꢀ1
nr f ꢀ1
a
b
d
r e ꢀ1
nr f ꢀ1
g
l
max /nm
F
PL /% Contribution /%
t /ms
k
/s
k
/s
l
max /nm
F
PL /% t /ms
k
/s
k
/s
DEST /eV
4
4
5
5
5
4
3
3
3
3
3
2
1
2
3
509
519
503
43
29
79
TADF(93) + Phos.(7)
TADF(89) + Phos.(11) 16
TADF(97) + Phos.(3) 5.5
5.5
8.0 ꢁ 10
1.8 ꢁ 10
1.4 ꢁ 10
1.0 ꢁ 10
4.4 ꢁ 10
3.8 ꢁ 10
523
546
516
67
54
82
158
356
209
4.2 ꢁ 10
1.5 ꢁ 10
3.9 ꢁ 10
2.1 ꢁ 10
1.3 ꢁ 10
8.4 ꢁ 10
0.089
0.132
0.094
a
b
c
Emission maximumd. Photoluminescence quantum yield (relative error ꢂ5%). Contribution of each of the paths (TADF and phosphorescence)
e
f
g
to the luminescence. Emission decay time (monoexponential fit, relative error ꢂ1%). Radiative decay rate. Nonradiative decay rate. Energy
gap between S and T determined from the emission spectra at 77 K and 298 K (see Fig. 5).
1
1
Fig. 6 (a) Temperature dependence of the decay time for the complex 1 crystal. (b) Temperature dependence of the decay time for the complex 2
crystal. (c) Temperature dependence of the decay time for the complex 3 crystal. The solid lines are the fitting curve according to eqn (1). (d) The
photoluminescence intensities of complex 1 stemming from the singlet state S
luminescence intensities of complex 2 stemming from the singlet state S (TADF) and the triplet state T
intensities of complex 3 stemming from the singlet state S (TADF) and the triplet state T (phosphorescence). It is relative to the total intensity Itot
I(S ) + I(T ) depending on temperature, according to eqn (2) and (3).
1
(TADF) and the triplet state T
1
(phosphorescence). (e) The photo-
1
1
(phosphorescence). (f) The photoluminescence
1
1
=
1
1
1
T states, the decay times and their relationship with tempera- determined from the onsets of the emission spectra at 77 K and
3
b,13–15
ture can be expressed by the following equation.
298 K (Table 1). Because of the small DEST, the lower energy T
excitons can convert gradually into the S state with an increase of the
temperature, which leads to the shortening of the radiative lifetime.
1
ꢀ
ꢁ
1
1
3
DEST
1
þ
exp ꢀ
kBT
tobs ¼
ꢀ
ꢁ
(1)
With this, the emissions of the complexes originate from both S and
1
1
1
DEST
kBT
þ
exp ꢀ
T1 states, i.e., emitting TADF and phosphorescence in the meantime.
tðT1Þ 3tðS1Þ
To make a rough estimate of the relative contributions of TADF
In this equation, k and T are the Boltzmann constant and the and phosphorescence at ambient temperature, we assume that
B
absolute temperature, respectively, and t(S
times of the S state and the T state, respectively. DEST is the energy using the FPL values obtained at 298 K and 77 K, respectively
gap between the S and the T excited states. The fitted results are (Table 1), and supposing that the populations of the S and T
1
) and t(T
1
) are the decay
1 1
FPL(S ) and FPL(T ) remain constant at varied temperature. By
1
1
1
1
1
1
presented in Fig. 6 and the red fit curve matches well with the states conform to a Boltzmann distribution (fast equilibration), the
6a,12,16
experimental results. The parameters acquired by fitting eqn (1) are relative intensities can be given by the following equation:
t(S ) = 73 ns, t(T ) = 155 ms, and DE = 0.090 eV for 1, t(S ) = 75 ns,
1
1
ST
1
t(T ) = 341 ms, and DE = 0.119 eV for 2, and t(S ) = 51 ns, t(T ) =
IðT Þ
IðT Þ
1
ST
1
1
1
1
¼
¼
2
07 ms, and DEST = 0.095 eV for 3. The fitted t(T ) values are close to
I
IðS Þ þ IðT Þ
1
tot
1
1
the values observed at 77 K, which are 158, 356 and 209 ms for the
complexes respectively. The fitted DEST values are in good agreement
with the values (0.089 eV for 1, 0.132 eV for 2 and 0.094 eV for 3)
(2)
ꢂ
ꢀ
ꢁꢃꢀ
1
FPLðS1ÞtðT1ÞgðS1Þ
FPLðT1ÞtðS1ÞgðT1Þ
DEST
kBT
1 þ
exp ꢀ
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IðS1Þ
Journal of Materials Chemistry C
ꢂ
ꢀ
ꢁꢃꢀ
1
FPLðS1ÞtðT1ÞgðS1Þ
FPLðT1ÞtðS1ÞgðT1Þ
DEST
kBT
PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonic acid) and ITO is indium tin oxide. Fig. 7a shows the
energy level diagrams and structure configurations of the
organic materials. The HOMO/LUMO levels of the studied
complexes are ꢀ5.20/ꢀ2.33 eV for 1, ꢀ5.21/ꢀ2.32 eV for 2
and ꢀ5.22/ꢀ2.33 eV for 3, respectively, which are calculated
from their electrochemical data and UV-vis data (see Fig. S3 and
Table S8 in the ESI†). The frontier orbital energy levels of the
¼
1 ꢀ 1 þ
exp ꢀ
(3)
Itot
In this equation, I(S
and the total intensity, and g(S ) = 1 and g(T ) = 3 are the
degeneracy factors for the singlet and the triplet states, respec-
tively. As shown in Fig. 6d–f, at temperatures below 90 K, there is
1 1 1 1
), I(T ) and Itot represent the intensity of S , T
1
1
only emission from the T
intensity contribution of phosphorescence reduces, which comes
with increasing contribution of the emission from the S state
TADF). At room temperature, the complexes mainly exhibit TADF
emissions, and the proportion of TADF is 93% for 1, 89% for 2 and
7% for 3, respectively. Comparatively speaking, complex 3 is the
1
state. As the temperature increases, the
3
f,17
others used in the devices are referenced to the literature.
1
In the OLEDs, PYD2 was used as the host material because
of its high triplet level (ꢀ2.93 eV) and good hole/electron-
transporting capability, and 3TPYMB was used as an electron-
transporting and hole-blocking material because of its outstanding
electron mobility, high triplet level, and deep HOMO energy level.
The hole-injection and emitting layers were prepared by spin
coating. The remaining layers were deposited by vacuum thermal
(
9
most efficient TADF material among the studied complexes.
Electroluminescence performance
To evaluate and compare the EL properties of these complexes, evaporation.
we used these complexes as emitting dopants to fabricate
The EL characteristics and numerical data of devices 1, 2
OLEDs through the solution process. The device architecture and 3 are depicted in Fig. 7 and Table 2. The excitons in three
was ITO (135 nm)/PEDOT:PSS (40 nm)/20 wt% 1, 2 or 3:80 wt% devices are recombined and confined in the emitting layer to
PYD2 (30 nm)/3TPYMB (50 nm)/LiF (0.8 nm)/Al (100 nm), where exhibit similar strong green emissions with a peak wavelength
Fig. 7 (a) Device configuration and related molecular structures of PYD2 and 3TPYMB. (b) Electroluminescence spectra of devices 1–3 containing 1–3,
respectively. (c) Current density–voltage–brightness (I–V–B) curves. (d) Plots of external quantum efficiency and current efficiency vs. current density.
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Table 2 Electroluminescence characteristics of OLEDs
ꢀ
1
EQE (%)
Max
CE (cd A
Max
)
Von (V)
ꢀ
2
ꢀ2
ꢀ2
ꢀ2
@100 cd m
@100 cd m
@1 cd m
Bmax (cd m
)
lmax (nm)
CIE1931 (x, y)
Device 1
Device 2
Device 3
7.6
6.2
8.3
7.5
5.9
8.1
24.8
20.4
27.1
24.5
19.4
26.2
8.5
7.7
7.3
2342 (22.9 V)
2012 (17.7 V)
2525 (21.7 V)
523
526
522
(0.300, 0.536)
(0.297, 0.538)
(0.292, 0.532)
3
1
at 523 nm for 1, 526 nm for 2 and 522 nm for 3. A maximum
P NMR. Elemental analyses (C, H, N) were carried out using an
ꢀ
2
brightness above 2000 cd m was observed for all the OLEDs. Elemental Vario EL III elemental analyzer. Thermogravimetric
No emissions from the host or other layers were observed. analysis (TGA) experiments were performed on a NETZSCH STA
The electroluminescence spectra agree well with those in the 449C Jupiter thermogravimetric analyzer in flowing nitrogen with
PYD2 film as shown in Table S5 in the ESI.† Complex 3 with a the sample heated in an Al
2
O
3
crucible at a heating rate of 10 K
ꢀ
1
carbazole appendage shows the highest device efficiency with min . The power X-ray diffraction (PXRD) data were collected at
ꢀ
1
a CEmax of 27.1 cd A and an EQEmax of 8.3%, which can be room temperature using a Rigaku MiniFlex 600 diffractometer
attributed to its high PLQY and the excellent hole transporting with Cu Ka radiation.
characteristics of the carbazole group. In the emitting layer,
Synthesis and characterization
this functional moiety helps recombination and transfer of the
charge carriers.
Synthesis of 6-methylpicolinohydrazide. Hydrazine hydrate
20 mmol, 1 mL) and methyl 6-methylpyridine-2-carboxylate
10 mmol, 1.512 g) were dissolved in ethanol (22 mL) and
(
(
Conclusions
heated under reflux for 6 h under air. After cooling down to
room temperature, the solvent was evaporated. The mixture
was washed with a small amount of ethanol and concentrated
to dryness to afford a light yellow solid (1.042 g, 69%).
Synthesis of 6,6 -(1H-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine).
A mixture of sodium (0.058 g) and 6-methylpicolinonitrile (7.5 mmol,
In conclusion, three new green emissive binuclear cuprous
complexes with different functional tetraimine ligands have
been designed and synthesized. The photoluminescent results
0
3
show that the functional substituents of CF and carbazole
groups have a very insignificant influence on the emission
energies of the studied Cu(I) complexes, but the influence of
different ligands on the efficiencies of their emissions is
evident. These complexes exhibit highly efficient TADF with a
very short emissive decay time at room temperature. Complex 3
with the carbazole group shows the highest efficiency in terms
of both photo- and electro-luminescence. The OLED with
complex 3 as an emitting dopant exhibits an EQE of 8.3%, a
0
.886 g) in anhydrous methanol was left undisturbed for 30 min.
Acetic acid (0.071 mL) and 6-methylpicolinohydrazide (6.8 mmol,
.031 g) were then added to the solution. The mixture was stirred
1
and refluxed until a yellow precipitate was formed. After filtration
and drying, the solid was heated at 200–210 1C for 4 h. The crude
product was purified by column chromatography on silica gel to
afford a white solid (1.486 g, 87%). H NMR (400 MHz, DMSO-d
1
6
):
d 14.82 (s, 1H), 8.03 (d, J = 7.6 Hz, 1H), 7.96 (dd, J = 14.6, 7.2 Hz, 2H),
.84 (t, J = 7.7 Hz, 1H), 7.46 (d, J = 7.6 Hz, 1H), 7.34 (d, J = 7.6 Hz, 1H),
.65 (s, 3H), 2.59 (s, 3H).
ꢀ
1
ꢀ2
CE of 27.1 cd A and a peak brightness of 2525 cd cm , while
complex 2 with a trifluoromethyl appendage exhibits poorer
quantum efficiency than the others. Due to their small activa-
7
2
Synthesis of the tetraimine ligands. Copper(I) iodide (0.52 mmol,
tion energy DE , short decay times, high efficiency, easy
ST
0
0.099 g), 6,6 -(1H-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine)
preparation and low cost, the binuclear Cu(I) complexes have
great potential for application in emitters. The results of this
study are interesting and are helpful in the design and devel-
opment of new efficient cuprous complexes that can be used in
highly efficient OLEDs.
(1.5 mmol, 0.377 g), 1,10-phenanthroline (0.95 mmol, 0.171 g),
caesium carbonate (2.17 mmol, 0.708 g) and 1.2 mmol iodobenzene
for pytzph, 1-iodo-4-(trifluoromethyl)benzene for pytzphcf or
9
-(4-iodophenyl)-9H-carbazole for pytzphcz were dissolved in
dry DMF (8 mL). The mixture was then stirred vigorously for
h at room temperature and then heated to 160 1C under N for
1
2
another 36 h. After cooling to room temperature, the solvent was
removed under reduced pressure. The residue was extracted and
Experimental
General procedures
2 2
washed with CH Cl and brine. The crude product was purified
All reactions were performed under a N
2
atmosphere using by column chromatography on silica gel to afford the products
standard Schlenk techniques unless specified otherwise. All as white powders.
0
chemicals were purchased from commercial sources and used
6,6 -(1-Phenyl-1,2,4-triazole-3,5-diyl)bis(2-methylpyridine)
1
without further purification. Solvents were freshly distilled over (pytzph). (0.192 g, 39%) H NMR (400 MHz, DMSO-d ): d 8.03
6
appropriate drying reagents. The complex [Cu(CH CN) ]BF
4
was prepared using the literature procedure. H and P NMR 7.34 (m, 2H), 2.60 (s, 3H), 2.25 (s, 3H).
(d, J = 7.7 Hz, 1H), 7.92–7.86 (m, 3H), 7.56–7.48 (m, 5H), 7.42–
3
4
18 1
31
0
spectra were recorded on a Bruker Avance III NMR spectrometer,
and 85% H PO
6,6 -(1-(4-(Trifluoromethyl)phenyl)-1,2,4-triazole-3,5-diyl)bis-
1
3
4
was used as the external standard substance in (2-methylpyridine) (pytzphcf). (0.288 g, 49%) H NMR (400 MHz,
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6
DMSO-d ): d 8.03 (dd, J = 15.1, 7.7 Hz, 2H), 7.98–7.86 (m, 4H), different temperatures were determined using a HORIBA
7.78 (d, J = 8.4 Hz, 2H), 7.45–7.36 (m, 2H), 2.61 (s, 3H), 2.20 Jobin-Yvon FluoroMax-4 instrument with multi-channel scaling
(d, J = 11.1 Hz, 3H).
(MCS) and time-correlated single-photon counting (TCSPC)
9
-(4-(3,5-Bis(6-methylpyridin-2-yl)-1,2,4-triazol-1-yl)phenyl)- peripheral equipment. The PL quantum yields of the solids
1
carbazole (pytzphcz). (0.321 g, 43%) H NMR (400 MHz, and films, which were defined as the number of photons
DMSO-d
6
): d 8.33 (d, J = 7.8 Hz, 2H), 8.08 (t, J = 7.4 Hz, 2H), emitted per photon absorbed by the system, were measured
7
.95 (dt, J = 19.3, 7.7 Hz, 2H), 7.86–7.78 (m, 4H), 7.57–7.47 using a HORIBA Jobin-Yvon FluoroMax-4-equipped with an
(
m, 4H), 7.46–7.34 (m, 4H), 2.63 (s, 3H), 2.36 (s, 3H).
integrating sphere.
Synthesis of the cuprous complexes. The three cuprous
complexes 1, 2 and 3 were prepared according to the following
X-ray crystallographic analysis
general procedure: a mixture of [Cu(CH CN) ]BF (2 mmol) and Diffraction data for the complexes were collected on a Rigaku
3
4
4
POP (2 mmol) in CH Cl (10 mL) was stirred at room tempera- Saturn 724HG CCD diffractometer equipped with graphite-
2
2
ture for 0.5 h, and then the corresponding tetraimine ligand monochromated Mo Ka radiation (l = 0.710747 Å) at room
1 mmol) was added. After stirring for another 2 h, the solvent temperature. The single crystals, measured using an X-ray
(
was evaporated. Single crystals of these products suitable for crystal diffractometer, were immersed in the mother liquor
X-ray diffraction measurements were obtained by diffusion of and sealed in a capillary glass tube. Structures were solved
ether into the CH using direct methods and refined using full-matrix least-
2 4 2 6
Cu (pytzph)(POP) ](BF ) (1). H NMR (400 MHz, DMSO-d ): squares methods with the SHELXL-97 program package.
2
Cl
2
solutions of the complexes.
1
[
2
d 8.19 (d, J = 7.4 Hz, 1H), 8.11 (t, J = 7.7 Hz, 1H), 7.60–7.24 Hydrogen atoms were added in idealized positions. The non-
(
(
(
m, 39H), 7.24–6.96 (m, 16H), 6.85 (d, J = 25.4 Hz, 6H), 6.74–6.65 hydrogen atoms were refined anisotropically. The SQUEEZE
3
1
m, 2H), 5.80 (s, 2H), 2.31 (s, 3H), 1.87 (s, 3H). P NMR routine in PLATON was used to remove highly disordered
162 MHz, DMSO-d
Cl Cu
C, 59.55 H, 4.28; N, 3.33.
Cu (pytzphcf)(POP) ](BF
6
): ꢀ12.70 (s), ꢀ18.41 (s). Anal. calcd for solvent molecules in complex 1. The final formulas were
C
95
H
79
B
2
6
2
F
8
N
5
O
2
P
4
: C, 58.22; H, 4.06; N, 3.57. Found: calculated according to the Squeeze results combined with
the results from elemental analyses and thermogravimetric
): analysis (TGA). Details of the crystal and structure refinements
1
[
2
2
4
)
2
(2). H NMR (400 MHz, DMSO-d
6
d 8.19 (s, 1H), 8.14 (d, J = 7.3 Hz, 1H), 7.91 (d, J = 7.3 Hz, 2H), for complexes 1–3 are listed in Table S1 (ESI†). Selected bond
.75–7.24 (m, 36H), 7.04 (ddd, J = 75.7, 47.4, 22.2 Hz, 22H), 6.69 lengths and bond angles are listed in Table S2 (ESI†). CCDC
7
3
1
(s, 2H), 5.80 (s, 2H), 2.27 (s, 3H), 1.92 (s, 3H). P NMR 1528952, 1528953 and 1528954 contain the supplementary
(
C
5
162 MHz, DMSO-d ): ꢀ12.54 (s), ꢀ18.22 (s). Anal. calcd for crystallographic data for complexes 1, 2 and 3, respectively.
6
98
H
82
B
2
Cl10Cu
2.92; H, 3.87; N, 3.20.
Cu (pytzphcz)(POP)
2
F
11
N
5
O
2
P
4
: C, 53.55; H, 3.76; N, 3.19. Found: C,
Computational methodology
1
[
2
2
](BF
4
)
2
(3). H NMR (400 MHz, DMSO-d
6
): Calculations on the electronic structures of 1–3 were carried out
d 8.31 (d, J = 7.8 Hz, 2H), 8.22 (s, 1H), 8.14 (t, J = 7.7 Hz, 1H), by using density functional theory (DFT) with the B3LYP hybrid
1
9
7
.79 (s, 2H), 7.69 (s, 2H), 7.60–7.26 (m, 40H), 7.25–6.81 functional. The input data were obtained from the X-ray
(
m, 23H), 6.69 (s, 2H), 5.80 (s, 1H), 2.38 (s, 3H), 1.94 (s, 3H). crystal structure. A ‘‘double-z’’ quality basis set consisting of
3
1
20
P NMR (162 MHz, DMSO-d ): ꢀ12.50 (s), ꢀ18.28 (s). Anal. Hay and Wadt’s effective core potentials (LANL2DZ) was
6
calcd for C H B Cl Cu F N O P : C, 57.04; H, 3.95; N, 3.66. employed for the Cu atom, and the all-electron basis set of
1
09 90
2
10
2 8 6 2 4
Found: C, 56.85; H, 4.12; N, 3.51.
6-31G* was used for P, O, N, C, F and H atoms. Time-dependent
DFT (TD-DFT) calculations were then performed on the opti-
mized structures at ground states by using the B3LYP func-
Electrochemical measurements
Cyclic voltammetry was performed in a gastight single- tional. The vertical excitation energies in the gas state were
compartment three-electrode cell using a BAS Epsilon Electro- obtained by calculating the 10 lowest singlet and 10 lowest
2
1
chemical Analyzer at room temperature. A glassy carbon disk triplet excited states. All calculations were carried out using
2
2
and a platinum wire were selected as the working and auxiliary Gaussian 03. The partition orbital composition was analysed
+
23
electrodes, respectively. The reference electrode was Ag/Ag
0.1 M of AgNO in CH Cl ). The CV measurements were carried
out in anhydrous and nitrogen-saturated CH Cl solutions with
.1 M n-tetrabutylammonium perchlorate (TBAP) and 2.0 mM The host material 2,6-bis(N-carbazolyl)pyridine (PYD2) was
by using the Multiwfn 2.4 program.
(
3
2
2
Device fabrication and characterization
2
2
0
2
4
analyte. The ferrocenium/ferrocene couple (0.58 V) was used as prepared according to the literature. The hole-blocking material
an internal standard (see Fig. S3 in the ESI†).
tris[3-(3-pyridyl)mesityl]borane (3TPYMB) and the host injecting
material poly(3,4-ethylene dioxythiophene):poly-(styrene sulfonic
acid) (PEDOT:PSS) were purchased from commercial sources.
Photophysical measurements
The UV-vis absorption spectra were recorded using a Perkin-Elmer PEDOT:PSS was filtered through a 0.22 mm filter before use. Other
Lambda 45 UV/vis spectrophotometer. Photoluminescence materials used in the device fabrication were purchased and used
spectra at both 298 K and 77 K were recorded on a HORIBA without further purification. A 40 nm thick PEDOT:PSS film was
Jobin-Yvon FluoroMax-4 SPECTROMETER. The lifetimes at first spin-coated (at 4800 rpm) on a pre-cleaned ITO-glass
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substrate and then dried at 125 1C for 25 min. The emitting
(b) Z. Wu, J. Luo, N. Sun, L. Zhu, H. Sun, L. Yu, D. Yang,
X. Qiao, J. Chen, C. Yang and D. Ma, Adv. Funct. Mater.,
2016, 26, 3306.
layer was then overlaid by spin coating (at 2100 rpm) a filtered
ꢀ1
5
.5 mg mL CH Cl solution with the host and dopant (2 mg of
2 2
complex 1, 2 or 3 and 8 mg of PYD2 dissolved in 1.8 mL
5 (a) Q. Zhang, X. L. Chen, J. Chen, X. Y. Wu, R. Yu and
C. Z. Lu, Dalton Trans., 2015, 44, 10022; (b) D. G. Cuttell,
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of CH Cl ). The films were then dried under vacuum for 3 h at
2
2
room temperature. Subsequently, 50 nm of 3TPYMB, 0.8 nm
of LiF, and 100 nm of Al were deposited thermally under a
ꢀ4
pressure of less than 4 ꢁ 10 Pa. The electroluminescence (EL)
spectra were recorded on a HORIBA Jobin-Yvon FluoroMax-4
spectrometer. The current density–voltage–brightness (I–V–B)
curves of the devices were recorded on a Keithley 2400/2000
source meter and a calibrated silicon photodiode. All measure-
ments on the devices were carried out at room temperature.
6 (a) B. Krummacher, M. K. Mathai, V. E. Choong, S. A.
Choulis, F. So and A. Winnacker, Org. Electron., 2006,
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Acknowledgements
This work was supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000),
Key Research Program of Frontier Sciences, CAS (QYZDJ-SSW-
SLH033), the National Natural Science Foundation of China
8
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